1. Field of the Invention
The present invention relates to a ceria-zirconia composite oxide, a method for producing the same, and a catalyst for purifying exhaust gas using the ceria-zirconia composite oxide.
2. Description of Related Art
In the related art, composite oxides containing various metal oxides have been utilized as a support, a co-catalyst, and the like for a catalyst for purifying exhaust gas. As a metal oxide in such a composite oxide, ceria is favorably used because ceria is capable of storing and releasing oxygen (that is, has an oxygen storage capacity) according to the oxygen partial pressure in the atmosphere. In addition, recently, various kinds of composite oxides containing ceria have been studied, and various ceria-zirconia composite oxides and methods for producing the same have been disclosed.
Japanese Patent Application Publication No. 2011-219329 (JP 2011-219329 A) discloses an example of a ceria-zirconia composite oxide containing a composite oxide of ceria and zirconia. According to this related art, a content ratio of cerium to zirconium in the composite oxide is set in a range from 43:57 to 48:52 by molar ratio ([cerium]:[zirconium]). In addition, an intensity ratio I(14/29) of a diffraction line at 2θ=14.5° to a diffraction line at 2θ=29° and an intensity ratio I(28/29) of a diffraction line at 2θ=28.5° to the diffraction line at 2θ=29°, which intensity ratios I(14/29) and I(28/29) are calculated from an X-ray diffraction pattern obtained by an X-ray diffraction measurement using CuKα after heating under a temperature condition of 1100° C. in air for 5 hours, respectively satisfy the following conditions: I(14/29)≥0.015; and I(28/29)≤0.08. According to this related art, it is possible to provide a ceria-zirconia composite oxide having high heat resistance and being capable of exhibiting excellent oxygen storage capacity even after exposure to a high temperature for a long period of time. However, recently, demand characteristics for a catalyst for purifying exhaust gas have been increasing. Therefore, there is required a ceria-zirconia composite oxide having sufficiently higher oxygen storage capacity (OSC) and sufficiently higher heat resistance at the same time and capable of exhibiting more sufficiently excellent oxygen storage capacity (OSC) even after exposure to a high temperature for a long period of time.
In addition, International Publication No. WO2012/105454 discloses a catalyst for purifying exhaust gas including: a ceria-zirconia composite oxide (A) having a pyrochlore structure; and a ceria-zirconia composite oxide (B) having a cubic crystal structure. At least a part of the ceria-zirconia composite oxide (A) is composited with the ceria-zirconia composite oxide (B). However, in the catalyst for purifying exhaust gas disclosed in International Publication No. WO2012/105454, surface structures of primary particles of the ceria-zirconia composite oxide (A) having a pyrochlore structure are unstable, an effect of suppressing deterioration of an oxygen storing material is not sufficient, oxygen absorbing and releasing performance after exposure to a high temperature is not always sufficient, and durability is not sufficient.